Disposition of intravenous diazepam in young men and women

Summary

The disposition of a single intravenous dose of diazepam (10 mg) was studied in 11 young, healthy subjects (6 males and 5 females on oral contraceptives). Plasma samples were obtained over 28 days and diazepam and N-desmethyldiazepam plasma concentrations and diazepam free fractions were determined. The salivary excretion of diazepam and N-desmethyldiazepam was studied over 72 h. A series of psychomotor performance tests were administered over the first 8 h. Interindividual variation in mean diazepam disposition over time is not principally related to variation in plasma protein binding; 93% of the variation in clearance is accounted for by variation in intrinsic clearance. Interindividual variation in diazepam disposition is modest but the plasma clearance of diazepam in women on oral contraceptives (median 14.0 ml/min) is significantly (p=0.004) less than in men (median 23.4 ml/min) and the area under the curve (AUC) of diazepam is highly correlated with the AUC of the principal active metabolite (r=0.90, p<0.001). The AUC of N-desmethyldiazepam (median 9.2 µg·h/ml) in women is greater (p=0.06) than in men (median 7.5 µg·h/ml). On chronic administration of diazepam, therefore, women taking oral contraceptives will have greater plasma concentrations per unit dose of both diazepam and N-desmethyldiazepam than men. The clearance of diazepam in control groups of 11 young men (median 23.8 ml/min) and 10 young women not taking oral contraceptives (median 26.8 ml/min) is not significantly different. Plasma and salivary concentratrions of diazepam are correlated (p<0.001) but the predictive value of this correlation is limited (r=0.70) since the ratio of salivary to plasma concentrations varies significantly over the day. The use of calculated free diazepam plasma concentrations does not improve the correlation (r=0.68) but the slope of this regression (1.00) is that predicted by theory.